Free unsaturated fatty acids as well as acylated unsaturated fatty acids present in the main lipid classes are susceptible to oxidation. Although less often mentioned, sterols and carotenoids as well as their esters should be added to this list of lipids prone to oxidation. Unsaturated fatty acids can be regrouped in three main families according to the position of the double bonds in their hydrocarbon chain: Omega-3, 6 and 9 families. Lipid peroxydation is caused by “Reactive Oxygen Species”. This includes the non-radicals: hydrogen peroxide and singlet oxygen, and the radicals: superoxide, hydroxyl, lipid peroxyl and lipid alkoxyl. In the human body the most important species involved in fatty acid oxidation are the highly reactive hydroxyl radical and singlet oxygen.
Since the reaction RH+O2 generation of free radicals, is thermodynamically difficult (activation energy of about 35 kcal/ml), the production of the first few radicals necessary to start the propagation reaction normally must occur by some catalytic means such as hydroperoxide decomposition, light and heat exposure and metal catalysis.
Three different mechanisms are able to induce lipid oxidations of which a first is autoxidation by free radical reaction where the oxidation process is initiated by hydroxyl radicals.
A second mechanism is photo-oxidation. As singlet oxygen (1O2) is highly electrophilic, it can react rapidly with unsaturated lipids but by a different mechanism than free radical autoxidation. In the presence of sensitizers (chlorophyll, porphyrins, myoglobin, riboflavin, bilirubin, erythrosine, rose bengal, methylene blue and many other drugs and dyes), a double bond interacts with singlet oxygen produced from O2 by light. Oxygen is added at either end of a carbon double bond which takes the trans configuration. Thus, one possible reaction of singlet O2 with a double bond between C12 and C13 of one fatty acid is to produce 12- and 13-hydroperoxides. The lifetime of singlet O2 in the hydrophobic cell membrane is greater than in aqueous solution. Furthermore, photo-oxidation is a quicker reaction than autoxidation since it was demonstrated that photo-oxidation of oleic acid can be 30,000 times quicker than autoxidation and for polyenes photo-oxidation can be 1,000-15,000 times quicker. Similar effects have been described in liposomes and in intact membranes. Thus a combination of photosensitizers with polyunsaturated lipids, as often it is the case in food supplements or nutraceuticals provide conditions extremely favourable to photo-oxidation. That is why all the oils in food products should be protected from light.
Oxygen in the singlet state can apparently interpose between a labile hydrogen to form a hydroperoxide directly —RH+O2═ROOH.
The chains of reactions can be terminated in several ways:    I. Two lipid radicals combine to form a dimer and eventually polymeric products;    II. Peroxyl radicals can undergo cyclization followed by decomposition of the cyclic compounds, oxyacids, and hydrocarbons;    III. Presence of chain-breaking antioxidants, which are themselves capable of forming radicals, unite with lipid radicals.
Photosensitized oxidation is efficiently inhibited by carotenoids and the main protective role played by these compounds takes place in green plants. The inhibitory mechanism is thought to be through an interference with the formation of singlet oxygen from the oxygen molecule. In contrast, tocopherols inhibit this oxidation by quenching the previously formed singlet oxygen, forming stable addition products. When such oxidation processes occur in food lipids, the result is rancidity and deterioration in product quality. Nutritive value is then reduced as a result of the removal of essential fatty acids and antioxidant nutrients. Some oxidation products are toxic as well. The overall nutritional significance of the oxidation on the losses of essential fatty acids that ensue, are normally relatively small in relation to the total dietary polyunsaturated fatty acids. More serious is the loss of the antioxidant nutrients, Vitamin E, various carotenes and Vitamin C that will not play their protective role once they get into the body.
The possibility that dietary cholesterol is also oxidized must be seriously considered, especially if the level of protective antioxidants is reduced in the diet as a result of the oxidation of polyunsaturated fatty acids. The reduction of dietary antioxidants itself may have some serious consequences in the body defences against reactive oxygen species of free radicals.